Field of the Invention
This invention relates to a seam for joining together the ends of woven fabric to form an endless belt. It is applicable more specifically to joining the ends of woven synthetic dryer fabrics as used in the dryer section of paper making machines and is particularly applicable to joining the ends of those dryer fabrics in at least the machine direction of the fabric.
Description of Prior Art
Industrial papermaking fabrics can be made flat and joined endless either during their manufacture or in the web processing and/or forming machine. In the later case they are called on-machine seamable fabrics. As an example, seam loops are formed at each of the transverse end of the papermaking fabric. To make the industrial papermaking fabric endless when drawn into the machine, the two transversal edge sections are brought together and the seam loops at the different transversal edge sections are interdigitated to form a common channel through which a seam pintle is pulled for joining the two transversal edge sections. In many cases each of the seam loops is formed at the respective transversal edge section of the fabric, by weaving of an MD-yarn with the CD-yarns, passing beyond the CD-yarn which is closest to the transversal edge section, by forming a loop and by weaving the MD-yarn back into the fabrics.
It is common practice to weave the dryer fabric as a long wide flat single piece and then join the ends on the machine to make an endless belt. There are several known methods of making the joint, or seam, as it is called. In one method, each end of the fabric is provided with a set of metal clipper type loops. In another method, metal or plastic loops are sewn into the ends which have been reinforced to prevent unraveling. In still another method, cross machine strands (weft) are removed near the ends of the fabric and the ends are folded back in such a way that MD-loops in the unwefted sections project. In each case the joint is completed when the array of loops at one end is intermeshed with the array of loops at the other end and brought into register to form a tubular passage through which a hinge pin or pintle wire is inserted.
The seam area is the weakest part of the fabric, because the tensile strength of the seam is significantly lower than the tensile strength of the body of the fabric. Further on, the seam area tends to cause hydraulic and/or topographic marking of the fibrous web formed and/or processed thereon due to the different structure of the seam area in regard to the body of the fabric.
In the past many attempts have been made to improve the tensile strength of the seam on the one hand and to reduce the marking of the seam on the other hand.
U.S. Pat. No. 6,332,480 B1 and its counterpart European published patent application EP 1 054 097 A1, for example, describe a seam construction where the MD-yarn forming a seam loop weaves back into the weave path of the adjacent MD-yarn. The seam construction of that solution exhibits several disadvantages caused by an extreme bending of the MD-yarns when forming the loops when weaving back into the weave path of the adjacent MD-yarn. The extreme bending of the MD-yarns forming the loops impacts an increased strain into these MD-yarns causing the loops one hand tend to tilt in an upright position out of the planes of the web and machine sides of the fabric and on the other hand to increase the spacing between the loops which are arranged along the transverse ends of the fabric. Such a tilting of the loops as well as the increase in spacing between the loops causes an increase in topographical marking. The increase in spacing between the loops further increases air permeability in the seam area in relation to the body of the fabric. This causes an increase in hydraulic marking. Due to increased strain such a seam construction further shows relatively weak tensile strength, because the back woven part of the MD-yarns can be pulled out relatively easily.
U.S. Pat. No. 4,095,622 describes a woven fabric and a method of forming a seam therein comprising a woven fabric as used for supporting a paper web on a paper making machine.